[mips] Improve encapsulation of the .MIPS.abiflags implementation and limit scope of related enums

Summary:
Follow on to r212519 to improve the encapsulation and limit the scope of the enums.

Also merged two very similar parser functions, fixed a bug where ASE's
were not being reported, and marked CPR1's as being 128-bit when MSA is
enabled.

Differential Revision: http://reviews.llvm.org/D4384